Anamorphic microlens arrays are arrays of elliptical or asymmetric shaped microlenses that provide selectively broader divergence of light in any one of two orthogonal directions. Their use as asymmetric diffusers in the backlights of LCD devices in televisions, notebooks, monitors and cell phones might lead to more efficient use of the available light. For example, US 2006/0239629 discusses backlight configurations for LCD devices with an asymmetric top diffuser. Light from a linear light source is transmitted through a light guide panel and outputted through a front face to a prismatic film. The prismatic film then redirects the light in a direction that is more normal to the LCD. An asymmetric diffuser is applied between the prismatic film and the LCD panel and aligned with respect to the prismatic film such that it spreads light more in a second direction than the first. The prismatic film may be a standard prismatic film such as a brightness enhancement film or a reversed prismatic film. The asymmetric diffuser may be a separate sheet disposed between the reversed prismatic film and the LCD panel or it may be laminated or produced directly on the smooth side of the prismatic film. US 2006/0239629 indicates that the asymmetric diffuser may be a holographic diffuser or may be prepared by a coating technique but does not go into details of the fabrication process. Furthermore, US 2006/0239629 does not teach the use of an anamorphic microlens array as an asymmetric diffuser.
U.S. Pat. No. 7,092,166 describes the use of anamorphic microlens arrays in projection screens to provide greater divergence in the horizontal than in the vertical viewing direction. It is indicated that improvements in the light intensity distribution may be obtained by having a mixed population of two different microlens elements in the array. However, no details are provided as to how such an array is to be fabricated. Furthermore, U.S. Pat. No. 7,092,166 does not teach the use of the microlens array in an LCD backlight unit.
U.S. Pat. No. 6,700,702 also describes the use of anamorphic microlens arrays in projection screens. It is indicated that a random close-packed array of microlenses is desirable for avoiding undesirable diffraction effects such as Moiré effects. It is also indicated that a typical size for the microlenses is between 20 and 120 micrometers. However, once again, no details are provided as to how the microlens array is to be fabricated. Furthermore, U.S. Pat. No. 6,700,702 does not teach the use of the microlens array in an LCD backlight unit. The placement of the anamorphic microlens array in an LCD backlight unit is very different from that in a projection display. In an LCD backlight unit, the anamorphic microlens array is placed between the light source and the LCD panel whereas in a projection display the anamorphic microlens array is part of a projection screen that is placed between an image forming panel and the viewer.
U.S. Pat. No. 6,675,863 describes a method for fabricating asymmetric diffusers. A coating of photosensitive medium is first applied to a substrate and cured. The cured photosensitive medium is subjected to an optical exposure to create the desired pattern and the exposed photosensitive medium is then developed. An elastomer formulation is applied on the developed photosensitive medium and cured to create an elastomer master. The pattern on the elastomer master is then transferred to a metal roller to produce seamless diffuser films on plastic substrates. Clearly, this is a complex process requiring a very large number of steps. Furthermore, because of the relatively slow reaction of the photosensitive medium to light and the physical separation that is required between the light source and the photosensitive medium during recording, the process is extremely sensitive to vibration and movement.
U.S. Pat. No. 5,932,342 also describes a method for fabricating asymmetric diffusers. A dispersion of optically clear ellipsoidal particles in an optically clear medium is formed into a film by extrusion or casting. The ellipsoidal particles are aligned during the extrusion or casting process to provide asymmetric diffusers. The approach provides a volumetric diffuser wherein the ellipsoidal particles are embedded within the volume of the optically clear medium. It is preferable to have a surface diffuser wherein the diffusing elements are present only on the surface of the diffuser film in order to maximize light transmission through the diffuser film.
US 2005/0058947 describes a method for fabricating optical microstructures such as microlenses by rotating a cylindrical platform that includes a radiation sensitive layer. The optical microstructures that are imaged in the radiation sensitive layer may be developed to provide a master for replicating microlenses. The approach is similar to that described in U.S. Pat. No. 6,675,863 and suffers from the disadvantages noted above. A simple method for fabricating an optical sheet suitable for use in LCD backlight units and projection screens containing on one side a close-packed random array of anamorphic microlenses is still needed.
U.S. patent application Ser. No. 11/561,244 filed Nov. 17, 2006 describes a simple low-cost process for fabricating relatively large area microlens arrays comprising close-packed hemispherical shaped microlenses. The process involves casting a solution of an organic soluble polymer in a volatile water-immiscible organic solvent having specific gravity greater than that of water on a suitable surface, exposing the cast solution to a humid environment and condensing water droplets on the cast solution, evaporating off the solvent and condensed water droplets from the cast composition to create a first structured film with hemispherical micro-cavities, coating a second fluid polymer composition over the first structured polymer film, curing the second fluid polymer film while it is still in contact with the first structured polymer film to render it solid and create a transparent second structured film comprising a first flat side and a second side with an array of microlenses formed thereon corresponding to the hemispherical micro-cavities of the first structured film and separating the second structured film with the microlens array from the first structured film. Although this process is relatively simple to implement, it does not provide a film containing anamorphic or asymmetric shaped microlenses. In spite of the foregoing efforts, there remains a need for new anamorphic microlens array films and methods of making such a film.